In the pre-press sector of graphic arts conventional silver halide materials are used for a number of different applications. Rather sensitive camera materials are used for obtaining screened images. Scan films are used for producing colour separations from multicolour originals. Phototype setting materials record the information fed to phototype- and image setters. Relative insensitive photographic materials serve as duplicating materials usually in a contact exposure process. Silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view. E.g. the commonly used developing agent hydroquinone is a rather unwanted ingredient because of its allergenic effects. The biodegradation of disposed Phenidone is too slow. Sulphite ions show a high COD (Chemical Oxygen Demand) and the resulting sulphate ions are harmful for e.g. concrete. As a consequence it is undesirable that depleted solutions of this kind would be discharged into the public sewerage; they have to be collected and destroyed by combustion, a cumbersome and expensive process.
In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
For instance, dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. These types of dry imaging elements also called heat mode materials offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor any other protection from ambient light is needed. Heat mode recording materials are disclosed in e.g. U.S. Pat. Nos. 4,123,309, 4,123,578, 4,157,412, 4,547,456 and PCT applications WO 88/04237 and WO 93/03928. A disadvantage of heat mode recording materials is their low sensitivity requiring powerful exposure means. Moreover the density forming element, usually carbon, limits the obtainable sensitivity by its own absorption.
Another dry imaging system known for quite a while is 3M's dry silver technology. It is a catalytic process whichcouples the light-capturing capability of silver halide to the image-forming capability of organic silver salts. Traditionally, silver halide, preferably silver bromide, is formed in situ by reacting silver behenate with bromide ions. The result of this process is the formation of very fine grains of silver bromide, less than 500 angstroms in diameter and positioned in catalytic proximity to the silver behenate. Exposure to light causes photolytic reduction at the silver bromide crystal (latent image formation) and provides a silver nucleus in position to permit catalysis of the reduction of the organic silver salt to silver metal at an elevated temperature thus producing a visual density. A disadvantage of this technology is that in the non-exposed areas silver halide remains which forms print-out silver on aging thereby increasing the minimal density possibly to an unacceptable level for some purposes. Details on the dry silver technology can be found in U.S. Pat. Nos. 3,457,075, 3,839,049, 4,260,677 and J. Phot. Sci., Vol. 41 (1993), p. 108.
It is an object of the present invention to provide an alternative process for producing an image which requires no wet processing steps.
It is a further object of the present invention to provide a process for the formation of an image with low background density and excellent line and dot quality, and requiring no intense radiation source.
Other objects of the invention will become apparent from the description hereafter.